Different petroleum production methods can be employed as appropriate. These are mainly:                primary production relative to the spontaneous production of petroleum by internal pressure in the reservoir;        secondary production where the internal pressure in the reservoir is maintained by water injection; and        tertiary production comprising different enhanced recovery methods.        
The appropriate method is selected as a function of the required investment (capex) and operational costs (opex) which are themselves related to the type of oil, the type of reservoir and to the viscosity of the oil.
The numerous enhanced oil recovery methods employed in an oil field have each their advantages. Among these methods, the following may be cited in particular:                re-injection of the gas produced;        injection of carbon dioxide with the additional purpose of fixing same to avoid the greenhouse effect;        injection of different solvents;        steam heating for heavy oils;        injection of bases for acid oils;        injection of surfactants;        in situ combustions by injecting oxygen;        biological methods with formation of bio-surfactants;        electric field diffusion methods etc.        
One of the most simple and effective methods comprises injecting a viscous aqueous solution.
The solution is made viscous by dissolution of water soluble polymers, and particularly polyacrylamides, xanthan gums, and more anecdotally, guar gums or cellulose ethers.
Polyacrylamides constitute the preferred additives since they have very high resistance to biodegradation while xanthan gums require the use of strong doses of toxic bactericides, and particularly formaldehyde.
In general, the enhanced oil recovery method by polymer injection mainly comprises:                dissolving the polyacrylamide, delivered in powder or emulsion form, at a high concentration of between 5 and 20 g/L for example, in water or a desoxygenated brine (to prevent its degradation);        injecting this mother solution into the water or brine injection pipeline and injecting the resulting mixture into the well under consideration.        
The particular objective is to maintain a constant viscosity during the injection of the water soluble polymer solution, so that the reservoir can be swept efficiently.
It is now known how to measure on-line and continuously the viscosity of the mother solution with some accuracy, particularly with Brookfield viscometres. This measurement is possible even if the solution is highly viscous and has a degree of heterogeneity, due particularly, on the one hand, to the presence of measurement-distorting gas bubbles and, on the other hand, due to the variation of the viscosity in the same conditions of concentration as a function of the salinity.
The variations in flow rates and salinities inherent in industrial processes make it necessary to measure the viscosity on injection.
This may be done by taking samples under API (American Petroleum Institute) conditions in order to prevent mechanical degradation due to the rapid decompression of the solution.
However, to the knowledge of the Applicant, there is no device in existence for measuring the viscosity at the time of injection, when the pressure in the pipeline generally varies between 50 and 250 bars in enhanced oil recovery processes.
Prior art viscometres include:                FORD or ZAHN cup viscometres, consistometres, falling ball viscometres working at atmospheric pressure;        Brookfield rotary viscometres which can be put on-line but in respect of which, above some pressures, mechanical deformation phenomena distort the measurement. They are used up to 50 bars, and exceptionally 70 bars of pressure;        vibration viscometres that rise to slightly higher pressures but in respect of which the shear of the non-Newtonian solution gives random results that cannot be correlated with the Brookfield viscosities measured in the laboratory;        oscillating piston viscometres that have the same characteristics as vibration viscometres;        capillary viscometres, discontinuous devices that measure very low speeds and therefore very low shears. Only the Brookfield device allows extrapolations to similar although higher shears.        
Documents US 2009/0090504 and U.S. Pat. No. 4,821,564 describe devices for measuring the viscosity of an injection solution by measuring a pressure drop. However, as the viscosity is measured upstream of the injection pump, these devices are not designed for use at the injection pressure but pressures much lower than 50 bars.
The problem the invention proposes to solve is that of perfecting an on-line device for measuring, in high pressure conditions of between 50 and 250 bars, viscosities below 1000 cps that can be correlated with the Brookfield viscosities measured in the laboratory also known as “Yield Viscosity”.